Thermoplastic copolyester elastomers have gained acceptance in many fields because of their outstanding physical properties which are unique in relation to other thermoplastic polymers. However, copolyester elastomers are flammable and this limits their usefulness for preparing electrical parts, wire coverings for telecommunications, optical fiber outer jacketing and other applications where fire retardant materials are needed.
Numerous halogenated organic compounds either alone or in combination with antimony trioxide have been tested or even recommended for use in polyester homopolymers or random copolyesters. Because of the relatively high flammability of multi-block copolyesters, the ease with which such copolyesters undergo degradation during melt processing, and the difficulty of retaining a useful amount of flexibility in the multi-block copolyesters in the presence of significant amounts of added materials, there still is a need for fully acceptable fire retardant multi-block copolyester compositions that are low smoke generating, high char forming and substantially nondripping when burned. The present invention provides flame retardant multi-block copolyester compositions which do not exhibit enhanced degradation and which substantially retain a useful amount of the flexibility of the starting polymer. Moreover, and most importantly, these flame retardant compositions are low smoke generating, high char forming and substantially nondripping when burned.
Recently, substantially nondripping flame retardant copolyesters have been developed by adding to the copolyester a combination of a flame retardant to resist burning and the drip suppressant fumed colloidal silica or an organophilic clay which is the reaction product of at least one quaternary ammonium salt with smectite clay. Such flame retardant, drip suppressant copolyester compositions are described in U.S. Pat. No. 4,521,557 to McKenna dated June 4, 1985 and U.S. application Ser. No. 605,550 to Shain filed Apr. 30, 1984, both assigned to E. I. du Pont de Nemours and Company. These flame retardant, substantially nondripping copolyester compositions referred to above are quite useful especially for coverings on optical fibers and wire since they are substantially nondripping when burned. However, the flame retardant nondripping multi-block copolyester compositions generate considerable amounts of smoke and only small amounts of char when they are burned. For many uses, for example plenum cable covering, the compositions should not only be flame retardant when burned but also generate minimum amounts of smoke which, of course, presents a serious hazard during a fire and causes secondary damage over a large area of the building. Further, the compositions should have high char formation. High char formation is beneficial because the char has enough integrity to remain in place, for example, on a bundle of wires, and functions as an insulator. The char keeps some of the heat of the external fire away from the wire bundle, minimizing its contribution to the flame and maximizing the length of time during which the wires perform their normal function. Many compositions have been rejected by manufacturers because of the large amounts of smoke generated and small amounts of char formation by the polymers when burned.
The present invention is directed to a novel copolyester composition that is especially useful for covering bundles of insulated telecommunication wires, e.g., optical fibers, with a flame-protective jacket material, the copolyester compositions of this invention, which are not only flame retardant and substantially nondripping but, in addition, such compositions have low smoke generating characteristics together with high char formation. The copolyester compositions of this invention can be made low smoke generating by the addition of a small amount of a char forming additive, and can do so without intumescing into a ceramic barrier.